Semiconductor devices in which an electrode portion of a semiconductor element and a conductive portion of a substrate are adhered to each other are widely being used. For the adhesion of the electrode portion of the semiconductor element and the conductive portion of the substrate, a conductive adhesive or soldering may be used. While conductive adhesive has the advantage of being able to adhere at lower temperatures than soldering, they have higher bulk resistance than solder. Accordingly, ways to decrease the resistance of conductive adhesive have been studied.
In conventional conductive adhesives, silver is used as a conductive filler. Silver, however, has a migration property, and the price of silver is rising. Accordingly, using copper as a conductive filler has been contemplated. A conductive adhesive in which copper is used is required to be adapted for curing the copper, which is easily oxidized, in an ambient atmosphere.
As an example of a paste in which copper is used as a conductive filler, a conductive copper paste has been disclosed which includes, as essential components, a copper powder having a predetermined particle size distribution and tap density, a thermosetting resin, an organic carboxylic acid, a chelate agent, and additionally polybutadiene (Patent Literature 1, claim 1 and paragraphs 0013, 0022).
The disclosure is aimed at providing a conductive copper paste suitable for a fine pitch-compatible through hole. The conductive copper paste is adapted for screen printing. In addition, the conductive copper paste has good conductivity comparable to that of a conductive silver paste, and anti-migration property (Patent Literature 1, paragraph 0008). Specific examples of the organic carboxylic acid that are listed include salicylic acid, benzoic acid, tartaric acid, citric acid, maleic acid, succinic acid, fumaric acid, and malonic acid (Patent Literature 1, paragraph 0018). These organic carboxylic acids are all solid at normal temperature.
A circuit substrate conductive paste has also been disclosed that has the following features. The circuit substrate conductive copper paste comprises a metal powder including copper, a compound including at least two (meth)acrylic groups, and a β-dicarbonyl compound, and includes substantially neither an azo compound nor peroxide (Patent Literature 2, claim 1). It is described that the circuit substrate conductive paste may include a compound having a flux activity (Patent Literature 2, paragraph 0014). As examples of the compound having a flux activity, aliphatic carboxylic acids such as oleic acid are listed (Patent Literature 2, paragraphs 0038, 0046).
A conductive copper paste composition has also been disclosed. The conductive copper paste composition contains: a prepolymer that has at least two hydroxyl groups per molecule and at least one tertiary amine; a copper powder; an amino resin: and a reducing agent. The conductive copper paste composition can be used for etching with an acidic etching solution (Patent Literature 3, claim 1). As examples of the reducing agent, unsaturated monocarboxylic acids of carbon number 12 to 23, such as oleic acid and linolic acid, are listed (Patent Literature 3, paragraph 0016 paragraph).
However, it has been discovered that these conductive copper pastes are disadvantageous in that: the specific resistance increases under a high-temperature and short-time curing condition (for example, 210° C. for 10 min); and the specific resistance of the conductive copper paste after curing varies greatly depending on the content of the copper powder.